1. Field of the Invention
The present invention relates to an electromagnetic coil drive device incorporating an electronic circuit for generating a pulse signal to intermittently supply a power supply voltage to an operation coil exciting an electromagnetic.
2. Description of the Prior Art
An electromagnetic coil drive device is employed, for example, in electromagnetic switches. It is desirable from the point of electrical power consumption, that when a movable core is to be attracted, a large current flow through an operation coil so that the movable core is attracted and that after attraction of the movable core, a small current flow through the operation coil so that the movable core is held at an attracted position.
In the above-described electromagnetic, when the power supply voltage has different values, for example, 100 V and 200 V, operation coils having different rated voltages are required in accordance with the respective values of the power supply voltage. However, provision of the operation coils having different rated voltages is not desirable from the point of production efficiency.
Japanese Patent Laid-open Application (Kokai) No. 62-145619 discloses an electromagnetic coil drive device in view of the above-described circumstances. The disclosed electromagnetic coil drive device comprises a full-wave rectifying circuit rectifying an AC power supply, a switching element switched by an on-off drive circuit to which a pulse signal is supplied, thereby applying an output voltage (as the power supply voltage) from the rectifying circuit to an operation coil, an integrating circuit integrating the power supply voltage, and a delay circuit starting a time counting operation when the voltage detected by a voltage detecting circuit for detecting the power supply voltage reaches a predetermined level. The electromagnetic coil drive device further comprises a closing oscillation circuit generating a saw-tooth wave, a closing comparing circuit comparing the saw-tooth wave supplied from the closing oscillation circuit with an integrated output from the integrating circuit to thereby generate a closing pulse signal, a holding oscillation circuit generating a saw-tooth wave having a smaller duty ratio than the saw-tooth wave generated by the closing oscillation circuit, a holding comparing circuit comparing the saw-tooth wave generated by the holding oscillation circuit with an integrated output from the integrating circuit to thereby generate a holding pulse signal having a smaller duty ratio than the closing pulse signal generated by the closing comparing circuit, and a switching circuit performing the switching operation so that when the electrical power is supplied, the closing pulse signal generated by the closing comparing circuit is supplied to the on-off drive circuit for switching the switching element and so that after elapse of the predetermined period of time counted by the delay circuit, the holding pulse signal generated by the holding comparing circuit is supplied to the on-off drive circuit, thereby switching the switching element.
The above-described circuit arrangement allows a large current to flow through the operation coil as the closing pulse signal is supplied to the on-off drive circuit when the electrical power is supplied to the operation coil. This circuit arrangement further allows a small current to flow through the operation coil as the holding pulse signal having a smaller duty ratio than the closing pulse signal is supplied to the on-off drive circuit at the time when the movable core is held at the attracted position. Additionally, when the power supply voltage is at a high level, the integrated output from the integrating circuit is also at a high level, both the value of the integrated output of the integrating circuit and the increasing speed thereof are increased to respective values larger than those in the case where the power supply voltage is low. As a result, the duty ratio of the closing pulse signal from the closing comparing circuit is reduced to a value smaller than that in the case where the power supply voltage is low, thereby overcoming the problem of different values of the power supply voltage.
The above-described prior art device, however, necessitates two oscillation circuits for the closing and the holding respectively, two comparing circuits, and a switching circuit switching the oscillation circuits and the comparing circuits. Thus an increased number of electronic parts complicates the circuit arrangement and increases the production cost of the device.